Irregular Plurals and Their Possessives

Regular plurals end in -s, and their possessive just adds an apostrophe after that s: the dogs' leashes. Irregular plurals break the pattern at the plural stage, not the possessive stage. Child becomes children, not childs, and once you have that irregular plural, it still needs to show possession somehow. English has only one tool for that job, apostrophe-s, so children's is correct even though children already looks plural. The apostrophe isn't marking a plural here. It's doing the same possessive work it always does, just attached to a noun that got irregular earlier in the process.

A second kind of irregularity comes from words borrowed whole from Latin or Greek, where the plural is a different ending rather than an added letter: criterion becomes criteria, phenomenon becomes phenomena, datum becomes data. Adding an English -s on top, as in criterias, double-marks the plural and breaks the word, because it already did its pluralizing the Latin way. The fix is to know the pair and pick the one the sentence needs: one criterion, several criteria.

A third group refuses to change shape at all: series, species, means. Singular and plural look identical, so you can't read the number off the word itself. You read it off the rest of the sentence, usually a verb or a quantity word nearby. This whole cluster sits next to subject-verb agreement, since picking the right form of an irregular plural often decides whether the verb that follows should be singular or plural too.